A discussion of the chemistry and functions of parabens and their potential effects on the human body.
Contents
Introduction
Function of Parabens in Products
Possible Effects on the Human Body
Parabens and Dermatitis
Parabens and Enzyme Functions
Potential Effects of Parabens on Estrogenic Activity
Parabens and Breast Cancer
Parabens and Infertility
Conclusion
References
Introduction Parabens have been used as antimicrobial agents in a wide variety of cosmetic, pharmaceutical and food products since the 1920s [1,2]. Due to their efficiency and [[#|low]] cost, manufacturers from these industries have included parabens in product formulations, and have thus increased our daily contact with this chemical [1]. In the past, parabens have been considered a safe preservative, however, only recently scientists have begun studying the chemical properties and effects of parabens found in these products [1]. Understanding the chemistry of parabens and their side effects can help determine the risks associated with this commonly used preservative. Synthesis and Properties of Parabens Parabens are the generic term for esters of p-hydroxybenzoic acid (PHBA), esterified at the C-4 position [1,2]. Parabens that are most frequently used as preservatives include methyl, ethyl, propyl, heptyl, and benzyl paraben. In the United States, methyl, ethyl, propyl, butyl, and heptyl parabens are permitted for use [3]. The synthesis of a paraben is a relatively straight-forward reaction with simple starting materials. Parabens are created by the esterification of p-hydoxybenzoic acid with the corresponding alcohol [3,4]. This reaction takes place in the presence of an acid catalyst, such as sulfuric acid [4]. The acid is neutralized with sodium hydroxide, and the product is crystallized by cooling [4]. The general chemical structure of a paraben is presented in Figure 1.
Parabens are slightly soluble in water, and are completely soluble in ether, alcohol, glycerine, and propylene glycol [4]. Parabens were originally prepared to replace two acidic pH preservatives, benzoic acid and salicylic acid. Parabens, in solutions of pH 3, 6, and 8, remain unchanged at room temperature for 25 weeks [3]. Parabens are generally used in the pH of 4.5 to 7.5, where they are considered most stable [6]. Most parabens, except for methyl paraben, have no distinctive odor present [3].
Function of Parabens in Products
The alkyl esters of parabens are used throughout pharmaceutical, cosmetic and food industries as preservatives [1,3,4]. Parabens are widely used as preservatives due to their effectiveness in fighting microbes and fungi [1,3,4]. Parabens are more effective in preventing yeast and mold growth, compared to bacteria growth [3]. When combined with heat, parabens can combat salmonella and yeast [3]. In regards to antimicrobial activity in bacteria, parabens are known for reducing gram-positive bacteria production more than gram-negative bacteria [3]. Methyl, ethyl, propyl, and butyl parabens are used in concentrations of 0.01-0.1% in cosmetic products and food products, while pharmaceutical formulations have paraben concentrations ranging from 0.1-0.3% [7]. All parabens are allowed concentrations up to .1% in food and up to 1% in cosmetics[1]. Generally, as the alkyl length increases, the antimicrobial activity increases and the water solubility decreases [1,2]. The use of parabens as antimicrobial agents in products first began in the mid-1920s by pharmaceutical companies [2]. Because these chemicals are so simple and yet so effective in preventing microbial growth, many industries have incorporated these compounds into products that we are exposed to on a regular basis. The amount of paraben dissolved in water is directly related to its preservative ability [2]. Figure 2 depicts the water solubility of methyl, ethyl, propyl, and butyl parabens, in addition to other useful chemical properties associated with the compounds. Thus, the alkyl groups with fewer carbon atoms present (especially the methyl group) are the most practical choices for the use in food [2]. This results from the increased water solubility among parabens with smaller alkyl groups [2]. For many years, parabens have been considered safe; the FDA approved methyl and propyl paraben, two of the most commonly used parabens in the US, as Generally Recogonized as Safe (GRAS) for direct addition to food [2]. Parabens have been used in food for more than 50 years and continue to permeate other food categories [4]. According to Soni et al., parabens are found in multiple foods including processed vegetables, baked goods, fats and oils, seasonings, sugar substitutes, coffee extracts, fruit juices, syrups, jams and jellies, pickles and olives, sauces, soft drinks, and frozen dairy products [2,3]. Concentrations of parabens in these foods are between 450 and 2000 ppm [2]. Based on a survey completed by the FDA, the use of parabens in food during the years 1960 to 1970 demonstrated a 30-fold increase [2]. In addition to reduced microbial activity, parabens are used as preservatives due to their activity in the neutral pH range. This is a rare feature of preservatives; most preservatives are not able to function in the neutral pH range [3]. Parabens are also effective preservatives in both acidic and basic pH. Antimicrobial activity of parabens occurs in pH range of 3 – 8 [3]. This is a very large pH range compared to other common preservatives, such as sodium benzoate. Sodium benzoate works in the range of 2.5 – 4.0, a much narrower range for antimicrobial activity [3]. As well as a greater functional pH range, parabens are very stable under a variety of conditions. They are resistant to cold and are stable in air [3]. Additionally, parabens do not degrade in heat and steam sterilization, which makes the esters preferable over other preservatives [3].
In addition to use in food, parabens are used as preservatives in nearly all cosmetic products. Propyl and methyl parabens are the most frequently utilized paraben preservative and are found in more than 13,200 cosmetic formulations [2]. Studies have also shown that 99% of leave on cosmetic products contain parabens, while 77% of rinse off cosmetic products contain parabens [1]. Common cosmetic products that contain parabens include shampoos and conditioners, lotions, foundations, eye makeup, creams, as well as many other products [2]. Parabens are the favored preservatives in this industry because they do not have any odor or taste [2, 3]. At the same time, parabens are favored because of the lack of product discoloration upon using the preservative [2]. The neutral pH alsoprevents hardening in cosmetic formulations [2]. Cosmetic industries favor the use parabens because of their biodegradability and extremely low cost [2].
Parabens are extremely abundant in the pharmaceutical industry [1, 2, 3]. Propyl paraben is the most commonly used paraben in this industry, due to its effective fungistats properties [2]. Parabens can be found in anesthetics, syrups, contraceptives, and suppositories [2]. Oftentimes, parabens are combined and added to products to produce more active preservatives [2,3].
Potential Effects on the Human Body
According to Singhal et al., parabens have very low toxicity [3]. Singhal et al. most likely make this assumption based on the large LD50 value in mice. Methyl and propyl paraben, two of the most commonly used parabens in products today, have an LD50 value greater than 8000 mg/kg in propylene glycol [3]. Upon entering the body, parabens are suspected to first be absorbed in the intestines, followed by the hydrolysis into PHBA in the liver, which is then excreted in urine [1, 3, 8]. They are generally considered safe because PHBA is less toxic than the parent compounds and the excretion process usually takes place within 24 hours [3,5]. Furthermore, Aubert et al.’s research demonstrates that parabens are quickly excreted through the urine and do not produce significant systemic exposure [9]. Along with intestinal absorption, parabens may be absorbed through the skin and mucosa [1, 10]. Some research also suggests that there are esterases present in the skin that aid in partially converting parabens into PHBA upon topical application [8].
Parabens and Dermatitis
Although parabens are excellent preservatives, the effects on the human body are still being investigated. There has been an on-going debate over whether or not parabens cause cancer or have other negative effects on the body. Because parabens are found in such minute concentrations, some research suggests that parabens do not cause systemic harmful effects but may cause some allergic reactions [7]. Parabens have been reported to cause allergic contact dermatitis and sensitivity due to the higher paraben concentrations found in topical products [11 ,12]. In many cases, the dermatitis is prolonged and treatment is delayed because patients nor health care providers suspect that parabens are the cause for dermatitis [7]. Those with allergic reactions to parabens have difficulty avoiding such reactions since there are no legal requirements for providing content found in the products on labels [12]. Additionally, allergic reactions have been reported after injections of drug formulations containing parabens [7]. Sensitization may occur as well if parabens are applied to damaged or broken skin [4]. According to a 1973 study conducted by the North American Contact Dermatits Group, in a group of 1200 individuals, about 3% of those involved with the study demonstrated delayed hypersensitivity to reactions to parabens [7]. This data shows that allergic reactions to parabens occurs in a small portion of the population.
Parabens and Enzyme Functions
In addition to rare cases of dermatitis, parabens may also impact the function of enzymes within the body [4]. According to Soni et al., research involving methyl and propyl paraben has demonstrated that these parabens are able to increase in the activity of dihydrofolate reductase, which converts dihydrofolate into tetrahydrofolate [4]. Dihydrofolate reductase is an important enzyme that is involved in cell replication [13]. Dihydrofolate reductase produces tetrahydrofolate, which is a necessary cofactor involved in the synthesis of amino acids, purines, and pyrimidines [14]. Thus, dihydrofolate reductase is an important enzyme that indirectly helps build DNA. Methyl and propyl paraben may produce conformational changes in this enzyme and increase its affinity for binding to dihydrofolate [4]. When parabens bind to proteins, the antifungal and antimicrobial properties of the compounds are no longer active [4]. The effects of the conformational changes associated with this protein are not well known, and the presence of parabens bound to the protein may have adverse side effects [4].
Effect of Parabens on Estrogenic Activity
It is well known and generally accepted that estrogen is a major contributor the development of breast cancer [1, 6, 15, 16]. According to Routledge et al., there has not been enough research on the potential risks of human exposure, and the level of exposure, to xenoestrogens, which are hormones that imitate estrogen [6]. Because we are exposed to many synthetic compounds in products, and there is no systematic study to analyze these compounds, people may be exposed to an unknown amount of estrogenic chemicals [6]. Only recently, the chemicals found in deodorants have been screened for toxicology and hormonal activity [15]. Using various tests involving methyl, ethyl, propyl, and butyl parabens, many reports have shown that these compounds are weakly estrogenic [15, 16].The greatest risk of breast cancer involves estrogen-progestagen hormone replacement therapy [15]. Based on the structures of parabens, these compounds are predicted to bind to the estrogen receptors and also show activity to estrogen as well as progesterone receptors [15]. The consistent use of underarm cosmetics and other products may result in long-term exposure to estrogenic potency of parabens, although relatively weak [15]. Experiments involving in vitro yeast based estrogen assays and in vivo uterotrophic studies in rats have also demonstrated that parabens are weakly estrogenic [6]. A major issue regarding the use of parabens revolves around the unknown combinative effect of parabens. In many cosmetic products, different parabens are added to a product in varying concentrations [10]. We don’t know, however, about the effects of the mixtures of parabens and the estrogenity associated with the mixtures [10]. Van Meeuwen et al. attempted to address this issue in his study via in vitro experiments and concluded that parabens have an additive estrogenic effect in a concentration addition model [10]. In comparison, Dagher et al. performed studies on the metabolism of parabens in breast cancer cells, also known as MCF7 cells, due to parabens’ known estrognic character [17]. Using MTT assays, Dagher et al. reported that parabens were stable and did not undergo hydrolysis in MCF7 cell homogenates [17]. This leads to the idea that the stability of parabens may lead to accumulation in breast cancer tissue [17]. The results show that methyl paraben was less toxic than butyl or benzyl paraben while butylparaben was reported to be the most potent of the parabens [17, 18]. This indicates that an increase in alkyl chain length leads to increased stability. To investigate the effects of parabens on estrogen receptors, Okubu et al. performed an experiment involving methyl, ethyl propyl butyl isopropyl and butyl parabens via assaying estrogen-receptor dependent proliferation of MCF-7 cells [19]. The results concluded that parabens have the ability to bind to estrogen receptors, ER, and mimic the effects of natural estrogen [19]. Okubu et al. also shows that parabens can increase estrogen receptor dependent cell growth, which ultimately has an impact on estrogen dependent gene expression [19]. Okubu et al. also note that the parabens could have endocrine disrupting functions, which can negatively affect cells [19]. Parabens have similar structures to alkylphenols, which are weakly estrogenic in vivo [20]. However, due to the rapid metabolism and excretion of parabens, only very high concentrations or an excessive dose of parabens, would be able to produce a significant estrogenic effect in tissues [20].
Parabens and Breast Cancer
With each passing year, there seems to be more reported cases of breast cancer. Recently, scientists have begun investigating the link between parabens and breast cancer [15]. One of the most well recognized and controversial articles in regards to the relationship between parabens and breast cancer is Darbre’s Concentrations of Parabens in Human Breast Tumours. Based on his research, Darbre determined that parabens can be found in human breast tumor tissue samples [1]. In this study, the concentrations of parabens in breast tumor samples from 20 patients were analyzed using HPLC and tandem mass spectrometry [1]. An average concentration of approximately 20 ng was found in the breast tumour samples [1]. Because parabens are used in underarm deodorants and antiperspirants as antimicrobial agents, Darbre suggests that the use of these products contributes to the parabens found in the tissue samples [1]. The report shows that methylparaben was the most abundant paraben, while benzylparaben was not found within any of the 20 tissue samples. Because methylparaben is one of the most common parabens used in topical products, Darbre attributes the use of these products to the presence of parabens in breast tumors. Similarly, there was no presence of benzylparaben found within the tissue, which Darbre attributed to the fact that benzylparaben is used in very few products [1]. While Darbre’s studies have opened the eyes of the scientific community in regards to the parabens and the causation of breast cancer, there is not enough research to support this claim and definitively say that parabens cause cancer [15]. This results from a few important points posed by Harvey et al. In Darbre’s study, only breast tumors were analyzed. Since we do not know the level of parabens in other tissue or normal breast tissue, one cannot say that breast cancer is induced by parabens without comparison of paraben concentrations in these control tissues [15]. Also, other chemicals used in underarm cosmetics may contribute to the increase in incidences of breast cancer, and other sources, such as oral consumption of parabens, may be a factor [15]. Based on Darbre et al.’s findings, it is suggested that the absorption from the underarm is the route of entry, and most likely results from the use of underarm cosmetics [1]. However, his study does not investigate the route of entry and the source needs to be confirmed and further investigated [15]. In his research, Darbre suggests that as the alkyl chain length increases, the octanol/water partition coefficient increases [1]. If the parabens enter breast tissue intact, they may accumulate in the body’s fatty, lipid-rich tissue [21]. However, his findings are opposite of the trend predicted. Methylparaben was found to be the most abundant paraben found in tissue, however, this paraben has the smallest chain length and thus the smallest partition coefficient [21]. While there are some issues with the methodology of Darbre’s research, there are a few, critical, take home points from his findings. It is important to note that detection of parabens in breast tumor samples is consistent with the idea that there is a connection between estrogenic compounds used in deodorants/antiperspirants and the incidence of breast cancer [15]. However, based on the data presented, the results alone do not suggest that the presence of these chemicals is the causation of tumors in the patients [15]. Further research on the link between cancer and parabens is required to fully understand the role these compounds may have on the etiology of cancer.
Parabens and Infertility
Another issue of major concern is the reproductive toxicity of parabens. Some male infertility is caused by exposure to xenobiotics [5]. Since parabens are xenobiotic, some scientists say there is a link between parabens and infertility [5]. Parabens, particularly methyl paraben, has been reported to decrease sperm mobility and act as endocrine disruptors, which may have harmful effects on the male reproductive system [5,22]. An experiment involving rodents determined that butyl and propyl paraben negatively impact the reproductive system as well as the synthesis of testosterone [5]. This is yet another example of how parabens may alter the hormonal balances within the body. Interestingly, some scientists are now looking into the possibilities of using parabens as a form of contraception [22]. Other studies have shown that methyl, ethyl, propyl, and butyl parabens are effective spermicides [5, 22]. Additional research is necessary to gain a better understanding of how parabens may reduce reproductive potential.
Conclusion
Recently, there has been much concern over the use of parabens as preservatives in foods, drugs, and cosmetics [1, 3, 4]. In some cases, parabens have been reported to cause dermatitis or other allergic reactions [7, 11, 12]. Parabens are considered weakly estrogenic and may adversely affect the male reproductive system as well as different enzymes found throughout the body [4, 5]. The presence of parabens has been determined in breast tumor tissue and the estrogenic nature of these compounds may be related to cancer, however, a more detailed investigation is required to determine how parabens get into tumors and their relationship to the disease [1,15]. Generally, the small concentrations that we encounter on a daily basis does not have a major impact on human health, however, there may be a risk upon the accumulation of these preservatives. With additional time and research, we will we ascertain the effects of prolonged exposure to the preservatives. Despite the mixed reviews about the safety of parabens, based on the current research and data presented, these compounds have potential to produce serious harm to the human body and should be reconsidered for use in the pharmaceutical, cosmetic, and food industries.
References
1. Darbre, P.D., Aljarrah, A., Miller, W.R., Coldham, N. G., Sauer, M.J. and Pope, G.S. “Concentrations of parabens in human breast tumours.” Journal of Applied Toxicology 24.1 (2004) : 5–13. [DOI] [KL Ref. 1.zip]
2. Soni, M.G., Carabin I.G., Burdock, G.A. “Safety assessment of esters of p-hydroxybenzoic acid (parabens).” Food and Chemical Toxicology 43.7 (2005) : 985-1015. [DOI]
4. Soni, M. G., Taylor, S.L., Greenberg N.A., and Burdock, G.A. “Evaluation of the health aspects of methyl paraben: a review of the published literature.” Food and Chemical Toxicology 40.10 (2002) : 1335-1373.[DOI]
5. Tavares, R.S., Martins, F.C., Oliveira, P.J., Ramalho-Santos, J., and Peixoto, F.P. “Parabens in male infertility—Is there a mitochondrial connection?” Reproductive Toxicology27.1 (2009) : 1-7. [DOI]
6. Routledge, E. J., Parker, J., Odum, J., Ashby, J., and Sumpter, J. P. “Some Alkyl Hydroxy Benzoate Preservatives (Parabens) Are Estrogenic.” Toxicology and Applied Pharmacology 153.1 (1998) : 12-19. [DOI]
7. Aronson, J.K., “Parabens” Meyler's Side Effects of Drugs: The International Encyclopedia of Adverse Drug Reactions and Interactions (Fifteenth Edition), Elsevier, Amsterdam(2006) : 2679. [DOI] [KL Ref. 7.zip]
8. Shaw, J. and deCatanzaro, D., “Estrogenicity of parabens revisited: Impact of parabens on early pregnancy and an uterotrophic assay in mice.” Reproductive Toxicology 28.1 (2009) : 26-31. [DOI]
9. Aubert, N., Ameller, T., and Legrand, J. “Systemic exposure to parabens: Pharmacokinetics, tissue distribution, execretion balance and plasma metabolites of [14C]-methyl-, propyl- and butylparaben in rats after oral, topical or subcutaneous administration.” Food and Chemical Toxicology50. 3–4 ( 2012) : 445-454. [DOI]
10. Van Meeuwen, J.A., van Son, O., Piersma, A.H., de Jong, P.C., and van den Berg, M. “Aromatase inhibiting and combined estrogenic effects of parabens and estrogenic effects of other additives in cosmetics.” Toxicology and Applied Pharmacology 230.3 (2008) : 372-382.[DOI]
11. Martins, I., Carreira, F.C., Canaes, L.S., de Souza Campos F.A., da Silva Cruz, L.M., and Rath, S. ”Determination of parabens in shampoo using high performance liquid chromatography with amperometric detection on a boron-doped diamond electrode.” Talanta85.1 (2011): 1-7. [DOI]
12. Schorr, W. F. “1743. Parabens again: Schorr, W. F. (1968). Paraben allergy: A cause of intractable dermatitis J. Am. med. Ass.204, 859.” Food and Cosmetics Toxicology7 (1969): 265. DOIKL Ref. 12.zip
13. Feeney, J. “NMR studies of interactions of ligands with dihydrofolate reductase.” Biochemical Pharmacology 40.1 (1990) : 141-152.[DOI]
14.Freudenthal, R., and Hebborn, P. “Studies on dihydrofolate reductase from normal and neoplastic mouse tissue, II. Effects of some organic solvents on the enzyme.” International Journal of Biochemistry 1.2 (1970) : 150-156. DOI
15. Harvey, P.W. and Everett, D. J. "Significance of the detection of esters of p-hydroxybenzoic acid (parabens) in human breast tumours." Journal of Applied Toxicology 24 (2004) : 1-4. [DOI] [KL Ref. 15.zip]
16. Byford, J.R., Shaw, L.E., Drew, M.G.B., Pope, G.S., Sauer, M.J., and Darbre, P.D. “Oestrogenic activity of parabens in MCF7 human breast cancer cells.” The Journal of Steroid Biochemistry and Molecular Biology80.1(2002) : 49-60. [DOI]
17. Dagher, Z., Borgie, M., Magdalou, J., Chahine, R., and Greige-Gerges, H.” p-Hydroxybenzoate esters metabolism in MCF7 breast cancer cells.” Food and Chemical Toxicology50.11 (2012) : 4109-4114. [DOI]
18. Prusakiewicz, J.J., Harville, H.M., Zhang, Y., Ackermann, C., and Voorman, R.L. “Parabens inhibit human skin estrogen sulfotransferase activity: Possible link to paraben estrogenic effects.” Toxicology232.3 (2007) : 248-256. [DOI]
19. Okubo, T., Yokoyama, Y., Kano, K., and Kano, I. “ ER-dependent estrogenic activity of parabens assessed by proliferation of human breast cancer MCF-7 cells and expression of ERα and PR.” Food and Chemical Toxicology39.12 (2001) : 1225-1232. [DOI]
20. Hossaini , A., Larsen, J. J., and Larsen, J.C. “Lack of oestrogenic effects of food preservatives (parabens) in uterotrophic assays.” Food and Chemical Toxicology38.4 (2000) : 319-323. [DOI]
21. Golden, R., Gandy, J., and Vollmer, G. “A review of the endocrine activity of parabens and implications for potential risks to human health.” Critical Reviews in Toxicology35. 5 (2005) : 435-438. [DOI] [KL Ref. 21.zip]
22. Bao-Liang, S., Hai-Ying, L., and Dun-Ren, P. “In vitro spermicidal activity of parabens against human spermatozoa.” Contraception39.3 (1989) : 331-335. [DOI][KL Ref. 22.zip]
Keisha Lemar
Chemistry 367 Final Paper
A discussion of the chemistry and functions of parabens and their potential effects on the human body.
Contents
Introduction
Function of Parabens in Products
Possible Effects on the Human Body
Parabens and Dermatitis
Parabens and Enzyme Functions
Potential Effects of Parabens on Estrogenic Activity
Parabens and Breast Cancer
Parabens and Infertility
Conclusion
References
IntroductionParabens have been used as antimicrobial agents in a wide variety of cosmetic, pharmaceutical and food products since the 1920s [1,2]. Due to their efficiency and [[#|low]] cost, manufacturers from these industries have included parabens in product formulations, and have thus increased our daily contact with this chemical [1]. In the past, parabens have been considered a safe preservative, however, only recently scientists have begun studying the chemical properties and effects of parabens found in these products [1]. Understanding the chemistry of parabens and their side effects can help determine the risks associated with this commonly used preservative.
Synthesis and Properties of Parabens
Parabens are the generic term for esters of p-hydroxybenzoic acid (PHBA), esterified at the C-4 position [1,2]. Parabens that are most frequently used as preservatives include methyl, ethyl, propyl, heptyl, and benzyl paraben. In the United States, methyl, ethyl, propyl, butyl, and heptyl parabens are permitted for use [3]. The synthesis of a paraben is a relatively straight-forward reaction with simple starting materials. Parabens are created by the esterification of p-hydoxybenzoic acid with the corresponding alcohol [3,4]. This reaction takes place in the presence of an acid catalyst, such as sulfuric acid [4]. The acid is neutralized with sodium hydroxide, and the product is crystallized by cooling [4]. The general chemical structure of a paraben is presented in Figure 1.
Parabens are slightly soluble in water, and are completely soluble in ether, alcohol, glycerine, and propylene glycol [4]. Parabens were originally prepared to replace two acidic pH preservatives, benzoic acid and salicylic acid. Parabens, in solutions of pH 3, 6, and 8, remain unchanged at room temperature for 25 weeks [3]. Parabens are generally used in the pH of 4.5 to 7.5, where they are considered most stable [6]. Most parabens, except for methyl paraben, have no distinctive odor present [3].
Function of Parabens in Products
The alkyl esters of parabens are used throughout pharmaceutical, cosmetic and food industries as preservatives [1,3,4]. Parabens are widely used as preservatives due to their effectiveness in fighting microbes and fungi [1,3,4]. Parabens are more effective in preventing yeast and mold growth, compared to bacteria growth [3]. When combined with heat, parabens can combat salmonella and yeast [3]. In regards to antimicrobial activity in bacteria, parabens are known for reducing gram-positive bacteria production more than gram-negative bacteria [3]. Methyl, ethyl, propyl, and butyl parabens are used in concentrations of 0.01-0.1% in cosmetic products and food products, while pharmaceutical formulations have paraben concentrations ranging from 0.1-0.3% [7]. All parabens are allowed concentrations up to .1% in food and up to 1% in cosmetics[1]. Generally, as the alkyl length increases, the antimicrobial activity increases and the water solubility decreases [1,2]. The use of parabens as antimicrobial agents in products first began in the mid-1920s by pharmaceutical companies [2]. Because these chemicals are so simple and yet so effective in preventing microbial growth, many industries have incorporated these compounds into products that we are exposed to on a regular basis. The amount of paraben dissolved in water is directly related to its preservative ability [2]. Figure 2 depicts the water solubility of methyl, ethyl, propyl, and butyl parabens, in addition to other useful chemical properties associated with the compounds.
Thus, the alkyl groups with fewer carbon atoms present (especially the methyl group) are the most practical choices for the use in food [2]. This results from the increased water solubility among parabens with smaller alkyl groups [2].
For many years, parabens have been considered safe; the FDA approved methyl and propyl paraben, two of the most commonly used parabens in the US, as Generally Recogonized as Safe (GRAS) for direct addition to food [2]. Parabens have been used in food for more than 50 years and continue to permeate other food categories [4]. According to Soni et al., parabens are found in multiple foods including processed vegetables, baked goods, fats and oils, seasonings, sugar substitutes, coffee extracts, fruit juices, syrups, jams and jellies, pickles and olives, sauces, soft drinks, and frozen dairy products [2,3]. Concentrations of parabens in these foods are between 450 and 2000 ppm [2]. Based on a survey completed by the FDA, the use of parabens in food during the years 1960 to 1970 demonstrated a 30-fold increase [2]. In addition to reduced microbial activity, parabens are used as preservatives due to their activity in the neutral pH range. This is a rare feature of preservatives; most preservatives are not able to function in the neutral pH range [3]. Parabens are also effective preservatives in both acidic and basic pH. Antimicrobial activity of parabens occurs in pH range of 3 – 8 [3]. This is a very large pH range compared to other common preservatives, such as sodium benzoate. Sodium benzoate works in the range of 2.5 – 4.0, a much narrower range for antimicrobial activity [3]. As well as a greater functional pH range, parabens are very stable under a variety of conditions. They are resistant to cold and are stable in air [3]. Additionally, parabens do not degrade in heat and steam sterilization, which makes the esters preferable over other preservatives [3].
In addition to use in food, parabens are used as preservatives in nearly all cosmetic products. Propyl and methyl parabens are the most frequently utilized paraben preservative and are found in more than 13,200 cosmetic formulations [2]. Studies have also shown that 99% of leave on cosmetic products contain parabens, while 77% of rinse off cosmetic products contain parabens [1]. Common cosmetic products that contain parabens include shampoos and conditioners, lotions, foundations, eye makeup, creams, as well as many other products [2]. Parabens are the favored preservatives in this industry because they do not have any odor or taste [2, 3]. At the same time, parabens are favored because of the lack of product discoloration upon using the preservative [2]. The neutral pH alsoprevents hardening in cosmetic formulations [2]. Cosmetic industries favor the use parabens because of their biodegradability and extremely low cost [2].
Parabens are extremely abundant in the pharmaceutical industry [1, 2, 3]. Propyl paraben is the most commonly used paraben in this industry, due to its effective fungistats properties [2]. Parabens can be found in anesthetics, syrups, contraceptives, and suppositories [2]. Oftentimes, parabens are combined and added to products to produce more active preservatives [2,3].
Potential Effects on the Human Body
According to Singhal et al., parabens have very low toxicity [3]. Singhal et al. most likely make this assumption based on the large LD50 value in mice. Methyl and propyl paraben, two of the most commonly used parabens in products today, have an LD50 value greater than 8000 mg/kg in propylene glycol [3]. Upon entering the body, parabens are suspected to first be absorbed in the intestines, followed by the hydrolysis into PHBA in the liver, which is then excreted in urine [1, 3, 8]. They are generally considered safe because PHBA is less toxic than the parent compounds and the excretion process usually takes place within 24 hours [3,5]. Furthermore, Aubert et al.’s research demonstrates that parabens are quickly excreted through the urine and do not produce significant systemic exposure [9]. Along with intestinal absorption, parabens may be absorbed through the skin and mucosa [1, 10]. Some research also suggests that there are esterases present in the skin that aid in partially converting parabens into PHBA upon topical application [8].
Parabens and Dermatitis
Although parabens are excellent preservatives, the effects on the human body are still being investigated. There has been an on-going debate over whether or not parabens cause cancer or have other negative effects on the body. Because parabens are found in such minute concentrations, some research suggests that parabens do not cause systemic harmful effects but may cause some allergic reactions [7]. Parabens have been reported to cause allergic contact dermatitis and sensitivity due to the higher paraben concentrations found in topical products [11 ,12]. In many cases, the dermatitis is prolonged and treatment is delayed because patients nor health care providers suspect that parabens are the cause for dermatitis [7]. Those with allergic reactions to parabens have difficulty avoiding such reactions since there are no legal requirements for providing content found in the products on labels [12]. Additionally, allergic reactions have been reported after injections of drug formulations containing parabens [7]. Sensitization may occur as well if parabens are applied to damaged or broken skin [4]. According to a 1973 study conducted by the North American Contact Dermatits Group, in a group of 1200 individuals, about 3% of those involved with the study demonstrated delayed hypersensitivity to reactions to parabens [7]. This data shows that allergic reactions to parabens occurs in a small portion of the population.
Parabens and Enzyme Functions
In addition to rare cases of dermatitis, parabens may also impact the function of enzymes within the body [4]. According to Soni et al., research involving methyl and propyl paraben has demonstrated that these parabens are able to increase in the activity of dihydrofolate reductase, which converts dihydrofolate into tetrahydrofolate [4]. Dihydrofolate reductase is an important enzyme that is involved in cell replication [13]. Dihydrofolate reductase produces tetrahydrofolate, which is a necessary cofactor involved in the synthesis of amino acids, purines, and pyrimidines [14]. Thus, dihydrofolate reductase is an important enzyme that indirectly helps build DNA. Methyl and propyl paraben may produce conformational changes in this enzyme and increase its affinity for binding to dihydrofolate [4]. When parabens bind to proteins, the antifungal and antimicrobial properties of the compounds are no longer active [4]. The effects of the conformational changes associated with this protein are not well known, and the presence of parabens bound to the protein may have adverse side effects [4].
Effect of Parabens on Estrogenic Activity
It is well known and generally accepted that estrogen is a major contributor the development of breast cancer [1, 6, 15, 16]. According to Routledge et al., there has not been enough research on the potential risks of human exposure, and the level of exposure, to xenoestrogens, which are hormones that imitate estrogen [6]. Because we are exposed to many synthetic compounds in products, and there is no systematic study to analyze these compounds, people may be exposed to an unknown amount of estrogenic chemicals [6]. Only recently, the chemicals found in deodorants have been screened for toxicology and hormonal activity [15]. Using various tests involving methyl, ethyl, propyl, and butyl parabens, many reports have shown that these compounds are weakly estrogenic [15, 16].The greatest risk of breast cancer involves estrogen-progestagen hormone replacement therapy [15]. Based on the structures of parabens, these compounds are predicted to bind to the estrogen receptors and also show activity to estrogen as well as progesterone receptors [15]. The consistent use of underarm cosmetics and other products may result in long-term exposure to estrogenic potency of parabens, although relatively weak [15]. Experiments involving in vitro yeast based estrogen assays and in vivo uterotrophic studies in rats have also demonstrated that parabens are weakly estrogenic [6]. A major issue regarding the use of parabens revolves around the unknown combinative effect of parabens. In many cosmetic products, different parabens are added to a product in varying concentrations [10]. We don’t know, however, about the effects of the mixtures of parabens and the estrogenity associated with the mixtures [10]. Van Meeuwen et al. attempted to address this issue in his study via in vitro experiments and concluded that parabens have an additive estrogenic effect in a concentration addition model [10]. In comparison, Dagher et al. performed studies on the metabolism of parabens in breast cancer cells, also known as MCF7 cells, due to parabens’ known estrognic character [17]. Using MTT assays, Dagher et al. reported that parabens were stable and did not undergo hydrolysis in MCF7 cell homogenates [17]. This leads to the idea that the stability of parabens may lead to accumulation in breast cancer tissue [17]. The results show that methyl paraben was less toxic than butyl or benzyl paraben while butylparaben was reported to be the most potent of the parabens [17, 18]. This indicates that an increase in alkyl chain length leads to increased stability. To investigate the effects of parabens on estrogen receptors, Okubu et al. performed an experiment involving methyl, ethyl propyl butyl isopropyl and butyl parabens via assaying estrogen-receptor dependent proliferation of MCF-7 cells [19]. The results concluded that parabens have the ability to bind to estrogen receptors, ER, and mimic the effects of natural estrogen [19]. Okubu et al. also shows that parabens can increase estrogen receptor dependent cell growth, which ultimately has an impact on estrogen dependent gene expression [19]. Okubu et al. also note that the parabens could have endocrine disrupting functions, which can negatively affect cells [19]. Parabens have similar structures to alkylphenols, which are weakly estrogenic in vivo [20]. However, due to the rapid metabolism and excretion of parabens, only very high concentrations or an excessive dose of parabens, would be able to produce a significant estrogenic effect in tissues [20].
Parabens and Breast Cancer
With each passing year, there seems to be more reported cases of breast cancer. Recently, scientists have begun investigating the link between parabens and breast cancer [15]. One of the most well recognized and controversial articles in regards to the relationship between parabens and breast cancer is Darbre’s Concentrations of Parabens in Human Breast Tumours. Based on his research, Darbre determined that parabens can be found in human breast tumor tissue samples [1]. In this study, the concentrations of parabens in breast tumor samples from 20 patients were analyzed using HPLC and tandem mass spectrometry [1]. An average concentration of approximately 20 ng was found in the breast tumour samples [1]. Because parabens are used in underarm deodorants and antiperspirants as antimicrobial agents, Darbre suggests that the use of these products contributes to the parabens found in the tissue samples [1]. The report shows that methylparaben was the most abundant paraben, while benzylparaben was not found within any of the 20 tissue samples. Because methylparaben is one of the most common parabens used in topical products, Darbre attributes the use of these products to the presence of parabens in breast tumors. Similarly, there was no presence of benzylparaben found within the tissue, which Darbre attributed to the fact that benzylparaben is used in very few products [1]. While Darbre’s studies have opened the eyes of the scientific community in regards to the parabens and the causation of breast cancer, there is not enough research to support this claim and definitively say that parabens cause cancer [15]. This results from a few important points posed by Harvey et al. In Darbre’s study, only breast tumors were analyzed. Since we do not know the level of parabens in other tissue or normal breast tissue, one cannot say that breast cancer is induced by parabens without comparison of paraben concentrations in these control tissues [15]. Also, other chemicals used in underarm cosmetics may contribute to the increase in incidences of breast cancer, and other sources, such as oral consumption of parabens, may be a factor [15]. Based on Darbre et al.’s findings, it is suggested that the absorption from the underarm is the route of entry, and most likely results from the use of underarm cosmetics [1]. However, his study does not investigate the route of entry and the source needs to be confirmed and further investigated [15]. In his research, Darbre suggests that as the alkyl chain length increases, the octanol/water partition coefficient increases [1]. If the parabens enter breast tissue intact, they may accumulate in the body’s fatty, lipid-rich tissue [21]. However, his findings are opposite of the trend predicted. Methylparaben was found to be the most abundant paraben found in tissue, however, this paraben has the smallest chain length and thus the smallest partition coefficient [21]. While there are some issues with the methodology of Darbre’s research, there are a few, critical, take home points from his findings. It is important to note that detection of parabens in breast tumor samples is consistent with the idea that there is a connection between estrogenic compounds used in deodorants/antiperspirants and the incidence of breast cancer [15]. However, based on the data presented, the results alone do not suggest that the presence of these chemicals is the causation of tumors in the patients [15]. Further research on the link between cancer and parabens is required to fully understand the role these compounds may have on the etiology of cancer.
Parabens and Infertility
Another issue of major concern is the reproductive toxicity of parabens. Some male infertility is caused by exposure to xenobiotics [5]. Since parabens are xenobiotic, some scientists say there is a link between parabens and infertility [5]. Parabens, particularly methyl paraben, has been reported to decrease sperm mobility and act as endocrine disruptors, which may have harmful effects on the male reproductive system [5,22]. An experiment involving rodents determined that butyl and propyl paraben negatively impact the reproductive system as well as the synthesis of testosterone [5]. This is yet another example of how parabens may alter the hormonal balances within the body. Interestingly, some scientists are now looking into the possibilities of using parabens as a form of contraception [22]. Other studies have shown that methyl, ethyl, propyl, and butyl parabens are effective spermicides [5, 22]. Additional research is necessary to gain a better understanding of how parabens may reduce reproductive potential.
Conclusion
Recently, there has been much concern over the use of parabens as preservatives in foods, drugs, and cosmetics [1, 3, 4]. In some cases, parabens have been reported to cause dermatitis or other allergic reactions [7, 11, 12]. Parabens are considered weakly estrogenic and may adversely affect the male reproductive system as well as different enzymes found throughout the body [4, 5]. The presence of parabens has been determined in breast tumor tissue and the estrogenic nature of these compounds may be related to cancer, however, a more detailed investigation is required to determine how parabens get into tumors and their relationship to the disease [1,15]. Generally, the small concentrations that we encounter on a daily basis does not have a major impact on human health, however, there may be a risk upon the accumulation of these preservatives. With additional time and research, we will we ascertain the effects of prolonged exposure to the preservatives. Despite the mixed reviews about the safety of parabens, based on the current research and data presented, these compounds have potential to produce serious harm to the human body and should be reconsidered for use in the pharmaceutical, cosmetic, and food industries.
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